1. Field of the Invention
This invention relates to an apparatus for receiving and decoding a signal such as that transmitted according to the digital video broadcast standard ("Specifications of the Baseline Modulation/Channel Coding System for Digital Multi-Programme Television by Satellite", European Broadcasting Union, January 1994). Such signals are commonly used in satellite communications systems which employ error correction to combat signal corruption. More specifically, this invention concerns a device with a variable rate QPSK/BPSK demodulator and a concatenated Viterbi/Reed-Solomon decoder.
2. Description of the Relevant Art
Digital broadcast satellite (DEBS) communication systems provide reliable long range transmission of information without the need for a pre-existing network of transmission lines and routing switches. However, since the costs entailed in creating a satellite and placing it in orbit are literally astronomical, the economic practicality of these systems depends in large part on widespread use of DBS receiver systems. Consequently containment of the costs for construction, distribution and maintenance of DBS receiver systems plays an important role for the emerging DBS technology.
FIG. 1 shows a standard model for a communications system 124 comprising a discrete-time channel 126 interposed between an encoder 128, and a decoder 130. Discrete-time channel 126 includes a continuous-time channel 140 interposed between a modulator-demodulator pair 138, 142. In this case the continuous-time channel may take the form of the atmosphere through which a broadcast signal propagates. The modulator-demodulator pair will typically use binary or quadrature phase shift keying as the modulation technique. By grouping the continuous-time channel with the modulator-demodulator pair, it becomes possible to treat the whole as a discrete-time channel which accepts a digital input signal and produces a possibly corrupted version of the input signal. Due to the power restrictions placed on satellite transmission channels, the probability of signal corruption is substantial.
To make satellite communications feasible, error correction codes are used which permit transmitted information to be communicated reliably at high data rates. The error correction coding scheme advocated by the standard referenced above is a concatenated coding scheme as shown in FIG. 1. Encoder 128 is comprised of three subcomponents: an outer encoder 134, an interleaver 136, and an inner encoder 132. Outer encoder 134 is a block encoder, in this case a Reed-Solomon encoder. Inner encoder 132 is a convolutional encoder. The combination of block and convolutional encoding is known to increase the error correcting capability of the decoder, and the use of an interleaver/de-interleaver pair serves to provide the large coding gain necessary to feasibly operate the power-limited satellite communications channel. Consequently, a critical part of the DBS receiver systems is the error correction decoding device. Hence it would be advantageous to provide a reliable yet economical implementation of an error correction decoding device as part of a DBS receiver system.